Field emission computer displays, in the general sense, are not new. For years there have been displays which comprise a plurality of field emission cathodes and corresponding anodes, the anodes emitting light in response to electron bombardment from corresponding the cathodes. Before entering a discussion on such displays, however, it is helpful to gain an understanding of the nature of field emission.
Field emission is a phenomenon which occurs when an electric field proximate the surface of an emission material narrows the width of a potential barrier existing at the surface of the emission material. This allows a quantum tunnelling effect to occur, whereby electrons cross through the potential barrier and are emitted from the material.
The field strength required to initiate emission of electrons from the surface of a particular material depends upon that material's "work function." Many materials have a positive work function and thus require a relatively intense electric field to bring about field emission. Some materials do, in fact, have a low, or even negative, work function and thus do not require intense fields for emission to occur. Such materials may be deposited as a thin film onto a conductor, resulting in a cathode with a relatively low threshold voltage required to produce electron emissions.
In prior art devices, it was desirable to enhance field emission of electrons by providing for a cathode geometry which focussed electron emission at a single, relatively sharp point at a tip of a conical cathode (called a micro-tip cathode). These micro-tip cathodes, in conjunction with extraction grids proximate the cathodes, have been in use for years in triode field emission displays.
For example, U.S. Pat. No. 4,857,799, which issued on Aug. 15, 1989, to Spindt et al., is directed to a matrix-addressed flat panel display using field emission cathodes. The cathodes are incorporated into the display backing structure, and energize corresponding cathodoluminescent areas on a face plate. The face plate is spaced 40 microns from the cathode arrangement in the preferred embodiment, and a vacuum is provided in the space between the plate and cathodes. Spacers in the form of legs interspersed among the pixels maintain the spacing, and electrical connections for the bases of the cathodes are diffused sections through the backing structure. Spindt et al. employ a plurality of micro-tip field emission cathodes in a matrix arrangement, the tips of the cathodes aligned with apertures in an extraction grid over the cathodes. With the addition of an anode over the extraction grid, the display described in Spindt et al. is a triode display.
Unfortunately, micro-tips employ a structure which is difficult to manufacture, since the micro-tips have fine geometries. Unless the micro-tips have a consistent geometry throughout the display, variations in emission from tip to tip will occur, resulting in unevenness in illumination of the display. Furthermore, since manufacturing tolerances are relatively tight, such micro-tip displays are expensive to make.
Another example of micro-tip cathodes is found in U.S. Pat. No. 5,038,070, which issued on Aug. 6, 1991 to Bardai et al., directed to a triode display and discloses a plurality of field emitters in the form of hollow, upstanding pointed cones or pyramids formed by a molding process. The plurality of field emitters extend from a surface of an electrically conductive layer. An electrically conductive mesh is adhered to an opposite surface of the conductive layer by a high temperature brazing process in electrical connection with the conductive layer. The mesh provides a strong metal base with good thermal conductivity for mounting. Additional elements such as a gate and anode structure may be formed on the conductive layer in alignment with the field emitters to form a field emitting triode array or the like.
A disadvantage of the field emitter structure taught in Bardai et al. is that emitter cones must be photolithographically grown, which is a very complex and expensive procedure.
Yet another triode micro-tip structure is illustrated in "Recent Developments on `Microtips` Display at LETI," published in the Technical Digest of IVMC, Nagahama, 1991. Author R. Meyer describes a micro-tip display having two salient features: (1) cold electron emission by field effect from a large matrix array of "micro-guns" (or micro-tips) and (2) low-voltage cathodoluminescence (of a few hundred volts). Again, Meyer uses micro-tip cathodes which have the disadvantages which have been noted above.
Another patent to Spindt et al., U.S. Pat. No. 5,015,912, which issued on May 14, 1991, teaches a matrix-addressed flat panel display using micro-tip cathodes of the field emission type. Spindt et al. discloses a grid structure for use in conjunction with micro-tip cathodes.
An attribute of the invention disclosed in Spindt et al. is that it provides its matrix-addressing scheme entirely within the cathode assembly. Each cathode includes a multitude of spaced-apart electron emitting tips which project upwardly therefrom toward a face structure. An electrically conductive gate or extraction electrode arrangement is positioned adjacent the tips to generate and control electron emission from the latter. Such arrangement is perpendicular to the base stripes and includes apertures through which electrons emitted by the tips may pass. The extraction electrode is addressed in conjunction with selected individual cathodes to produce emission from the selected individual cathodes. The grid-cathode arrangement is necessary in micro-tip cathodes constructed of tungsten, molybdenum or silicon, because the extraction field necessary to cause emission of electrons exceeds 50 Mv/m. Thus, the grid must be placed close (within approximately 1 micrometer) to the micro-tip cathodes. These tight tolerances require that the gate electrodes be produced by optical lithographic techniques on an electrical insulating layer which electrically separates the gates of each pixel from the common base. Such photolithography is expensive and difficult to accomplish with the accuracy required to produce such a display, thereby raising rejection rates for completed displays. Moreover, the extraction grid taught in Spindt et al. was specifically designed to operate in conjunction with micro-tip cathodes, and not with other geometries.
The two major problems with the device disclosed in Spindt et al. are 1) formation of the micro-tip cathodes and 2) formation and alignment of the extraction electrodes with respect to the cathodes. The structure disclosed in Spindt et al. is extremely intricate and difficult to fabricate in the case of large area displays.
The prior art has been directed to micro-tip cathodes, even in view of their formidable manufacturing difficulties, because they are advantageously used with an extraction grid in a triode (three terminal) structure.
In a triode (three terminal) pixel structure, an electron extraction grid structure is interspersed between corresponding cathode and anode pairs. In the case of triode displays, the grid gives an extra control parameter which produces several advantages. First, the grid can be controlled independent of the cathodes and anodes to thereby produce independently controllable cathode-anode and cathode-grid electric fields. This allows use of a very low control voltage to be applied to the cathode-grid field to effect electron emission, while the grid-anode voltage can be very high (several hundred to several thousand volts) to thereby result in higher power efficiency of the display. This is so because the anode phosphor material can be excited by electrons falling through a greater potential and, hence, be struck by electrons having a greater kinetic energy. Second, voltages selectively applied to address and excite individual grid-anode pairs can be lower (on the order of 40 volts), thereby allowing use of more conventional electronics in drive circuitry. Finally, the lower electric field between the grid and the anode (on the order of 1-5 volts per micrometer) reduces dielectric requirements for spacer material used to separate cathode and anode assemblies. Prior art extraction grid structures were designed to cooperate with micro-tip cathodes to enhance control of electron extraction and emission.
In Ser. No. 07/851,701, which was filed on Mar. 16, 1992, now abandoned and entitled "Flat Panel Display Based on Diamond Thin Films," an alternative cathode structure was first disclosed. The Ser. No. 07/851,701 discloses a cathode having a relatively flat emission surface. The cathode, in its preferred embodiment, employs an emission material having a relatively low effective work function. The material is deposited over a conductive layer and forms a plurality of emission sites, each of which can field-emit electrons in the presence of a relatively low intensity electric field.
Flat cathodes are much less expensive and difficult to produce in quantity because the fine, micro-tip geometry has been eliminated. The advantages of the flat cathode structure was discussed at length therein. The entirety of that application, which is commonly assigned with the present invention, is incorporated herein by reference.
A relatively recent development in the field of materials science has been the discovery of amorphic diamond. The structure and characteristics of amorphic diamond are discussed at length in "Thin-Film Diamond," published in the Texas Journal of Science, vol. 41, no. 4, 1989, by C. Collins et al., the entirety of which is incorporated herein by reference. Collins et al. describe a method of producing amorphic diamond film by a laser deposition technique. As described therein, amorphic diamond comprises a plurality of micro-crystallites, each of which has a particular structure dependent upon the method of preparation of the film. The manner in which these micro-crystallites are formed and their particular properties are not entirely understood.
Diamond has a negative electron affinity in the (111) direction. Thus n-type diamond has a negative work function. That is, only a relatively low electric field is required to distort the potential barrier present at the surface of diamond. Thus, diamond is a very desirable material for use in conjunction with field emission cathodes. In fact, the prior art has employed diamond films to advantage as an emission surface on micro-tip cathodes. However, the prior art has failed to recognize that amorphic diamond, which has physical qualities which differ substantially from other forms of diamond, makes a particularly good emission material. Ser. No. 07/851,701, now abandoned, was the first to disclose use of amorphic diamond film as an emission material. In fact, in the preferred embodiment of the invention described therein, amorphic diamond film was used in conjunction with a flat cathode structure to result in a radically different field emission cathode design. The micro-crystallites present in the amorphic diamond film are more or less disposed to function as electron emission sites, depending upon their individual structure. Therefore, over the surface of a relatively flat cathode emission surface, amorphic diamond micro-crystallites will be distributed about the surface, a percentage of which will act as localized electron emission sites.
The prior art has been entirely directed to triode flat panel displays based on micro-tip cathodes constructed of molybdenum, tungsten, silicon or similar materials. The prior art has failed to provide a matrix-addressable flat panel display that is 1) relatively simple in design, 2) relatively inexpensive to manufacture and 3) uses a triode (three terminal) pixel structure employing a cathode which has a relatively flat emission surface comprising a plurality of distributed localized electron emission sites.
The prior art has also failed to address the problem of providing an appropriate grid structure for use in conjunction with flat cathodes.
The purpose of the present invention is to build on the idea of depositing amorphic diamond film on the surface of relatively flat field emission cathodes, by providing a triode display structure employing a novel extraction grid proximate the flat cathodes to cause emission therefrom.